(a) Field of the Invention
The present invention relates to a high performance telephoto lens system having a large relative aperture ratio and corrected chromatic aberration due to the secondary spectrum.
(b) Description of the Prior Art
Recently, large aperture ratios such as F/2.0 or F/2.8, high photographing performance and shorter minimum photographing destance as well as light weight and high operability for focusing are desired even for telephoto lens systems having focal lengths on the order of 200 mm to 300 mm.
As lens systems so designed as to satisfy the desire, there have conventionally been known lens systems which comprise lenses made of a glass material having an especially low dispersing power and adopts an inner focusing system which performs focusing by shifting certain lens(es) in the lens system. As an example of these lens systems, there exists Japanese patent application No. 125218/81 (Japanese unexamined published patent application No. 27115/83) of the inventor now U.S. Pat. No. 4,457,595. This lens system is a high performance, larger aperture ratio type adopting a rear focus system utilizing a floating method (preventing variation of aberration by moving specific lens components independently of other lens components) in which three lens components (two lens components and a lens group) arranged on the rear side are shifted for focusing. This lens system has made it possible to perform focusing on objects located at infinite distance to a short distance corresponding to a photographing magnification level on the order of 1/10 with little variations of aberrations caused by moving said lens components.
This lens system uses a glass material having anomalous dispersion ratio for eliminating the secondary spectrum. However, a high degree of longitudinal chromatic aberration due to the secondary spectrum still remains in the said lens system, and it can not fully exhibit yet the effect of the focusing system which functions to correct aberrations produced by varying distance to an object to be photographed.
In order to correct the longitudinal chromatic aberration due to the secondary spectrum, it is sufficient to make the image point free from chromatic aberration. When C line and F line are made achromatic considering rays coming from infinite distance in a lens system composed of thin lens elements, chromatic aberration of g line is expressed by the following formula: ##EQU1## wherein the reference symbol f represents focal length of the lens system as a whole, the reference symbol .nu..sub.i designates Abbe's number of ith lens component, the reference symbol .theta..sub.i denotes partial dispersion ratio ##EQU2## the reference symbol h.sub.i represents height of incidence on the ith lens surface when that on the first lens surface is taken as 1, and reference symbol f.sub.i designates focal length of ith lens component.
Since .DELTA.Sg generally has a positive value, positive lens components should have large .theta..sub.i, whereas negative lens components should have small .theta..sub.i even when they are made of glass materials having the same Abbe's number. Further, it is effective for correction of said chromatic aberration to use such special glass materials for lens components having h.sub.i.sup.2 /f.sub.i as large as possible.
The lens system disclosed by the above-mentioned Japanese unexamined published patent application No. 27115/83 has a lens composition similar to that of the lens system according to the present invention.
Speaking concretely, either of the lens systems comprises a first converging lens group, a diverging lens group and a second converging lens group. Further, said first converging lens group comprises a positive lens component, a positive lens component, a negative lens component and a positive lens component; the second and third positive lens components being made of an anomalous dispersion glass material having a partial dispersion ratio larger than those of general glass materials. However, the first positive lens component having the largest h.sub.i.sup.2 /f.sub.i out of the lens elements arranged in the first converging lens group and the negative lens component are made of the general glass materials. As a result, it is difficult to minimize .DELTA.Sg in the lens system though it uses the anomalous partial dispersion glass material which requires high material cost and machining cost.